Aluminum brazing is usually accomplished by heating with a torch or other localized heat source, by salt-dip brazing, or in a furnace. Furnace brazing can be performed in air using active flux salts such as zinc chloride, however preferred furnace brazing processes use protective atmospheres such as vacuum, or inert gas, in combination with either fluxless braze promoters, or non-corrosive fluxes. Sometimes furnace brazing is used to assemble one set of components, and then additional components are brazed afterwards, using a secondary brazing operation that may use a localized heating method to avoid damage to the first brazed assembly. To braze aluminum, filler metals are normally used in the form of (1) wire or shim stock, (2) a paste of flux and filler metal powder or (3) a clad layer on brazing sheet composite.
Processes for brazing usually provide at least one mating surface having a specific bonding material, placing the mating surfaces in contact, and then applying a particular heating procedure to bring the assembly to a temperature range suitable to accomplish melting of the filler metals, and upon cooling, joining of the assembled components. Either a flux or a braze promoter is provided, typically in the filler metal, or applied to the filler metal surface, to permit disruption of surface oxides and wetting of the members to be joined by the filler metal.
Various methods of bonding aluminum are known in the prior art. In the case of complex assemblies such as heat exchangers, where multiple, thin-walled aluminum components are required to be sealingly joined with multiple braze bonds, furnace brazing processes have been most widely used. Because of the difficulty of post-braze removal of corrosive fluxes or salts, two general categories of furnace brazing have been most widely commercialized, ie, fluxless vacuum brazing (VB), and controlled atmosphere brazing (CAB).
In VB, the parts to be brazed are provided with sufficient quantities of magnesium, normally as magnesium alloy constituents in the filler metal or in the aluminum components, such that, when brought to temperature in a brazing furnace under sufficient vacuum conditions, the magnesium becomes sufficiently volatile to disrupt the oxide layer present and permit the underlying aluminum filler metal to flow together. While this technique provides for good bonding, it is essentially a discontinuous process, resultant from the need to apply a vacuum, and thus, is relatively expensive. It is also difficult to control, as it is very sensitive to oxidizing conditions in the furnace atmosphere, and demands that onerous standards of material cleanliness be maintained. Further, the evaporation of the magnesium leads to condensation in the brazing furnace, which requires frequent removal, thereby further adding to costs. For heat exchanger applications, it is sometimes desirable to add small amounts of zinc to the aluminum materials being brazed, to improve corrosion resistance. A limitation of VB however, is that the zinc constituents are, like magnesium, relatively volatile, so that control of the as-brazed zinc composition in the aluminum structure being brazed, is difficult.
In CAB, the ability to braze does not result from mechanical disruption of the oxide but rather from chemical modification of the oxide by a fluoride salt flux which is applied to the parts. An example of the type of flux used for CAB is NOCOLOK™ flux. As the name suggests, CAB does not require that a vacuum be drawn, such that the process may readily be carried out on a continuous basis, most typically using an inert gas furnace. While this provides for some reduction in cost, this cost saving is partially offset by the necessity for integration of flux application systems, many of which suffer from variable flux loading. Moreover, after the flux has been applied, the flux can be susceptible to flaking, such that braze quality is affected, or contamination of the article of manufacture can occur. The flux can also be difficult to apply, especially on internal joints; and can cause problems in terms of furnace corrosion and cleanliness in the finished product. More importantly however, it has been found that the flux can lose activity when exposed to magnesium. Thus, this process is not suitable for brazing magnesium-enriched aluminum alloys. As magnesium is a commonly used alloying element in aluminum to improve strength and/or other properties, this reduces the attractiveness of CAB.
Applications for brazing aluminum are not limited to heat exchangers. However, heat exchangers may comprise relatively complex assemblies of stacked plates or tubular members that require reliable, low cost joining of multiple joints. Some heat exchangers, for example oil coolers and air conditioning evaporators, require extensive brazing of internal joints while ensuring that the internal passageways of such heat exchangers do not provide a source for particulate flux residues in the functional lubrication or refrigerant system. Recently, stacked assemblies of brazed metal plates have been considered for use in the assembly of fuel cell engines. Because of their structural similarity to plate-type heat exchangers, heat exchanger brazing technology is of significant interest in the manufacture of fuel cell engines. The joining of fuel cell plates requires reliable laminar type bonds (extended lap joints). However, fuel cell plates tend to be thin and have intricately formed, narrow flow field channels that are easily clogged by flux or by excess filler metal flow. Using prior art CAB processes, it has been difficult to satisfactorily braze fuel cell plates without internal flux contamination, and therefore CAB is unattractive, and the cost of VB is prohibitive. As a consequence, fluxless brazing methods are of increased recent interest, for both heat exchanger and fuel cell engine applications.
A number of brazing processes disclosed in the prior art utilize filler metal compositions based on aluminum, zinc and silicon. For example, U.S. Pat. No. 5,464,146 discloses the deposition of a thin film of aluminum eutectic forming material (Si, Al—Si or Al—Zn), by electron beam physical vapor deposition or conventional sputtering on at least one of the shapes to be brazed or joined. The assembly is then heated to a temperature between 1075 and 1105° F. (579-596° C.) in the presence of a suitable fluxing agent, to diffuse eutectic forming material into the aluminum and form a braze joint.
U.S. Pat. No. 5,072,789, describes an aluminum heat exchanger with an aluminum fin and tube joined primarily by a fillet of zinc prepared using a zinc chloride slurry or zinc wire sprayed coating, again in the presence of a suitable flux. U.S. Pat. No. 4,901,908 describes a process of forming a zinc or zinc-aluminum alloy on an aluminum surface by a spraying technique, which alloy has a melting point lower than that of the core. In U.S. Pat. No. 4,890,784, diffusion bonding of aluminum alloys is performed using a thin alloy interlayer of magnesium, copper or zinc placed between mating surfaces of the alloy members to be bonded.
U.S. Pat. No. 4,785,092 discloses an aluminum clad brazing material consisting of 4.5 to 13.5% silicon, 0.005 to less than 0.1% strontium, and additionally one element from the group consisting of 0.3 to 3.0% magnesium, 2.3 to 4.7% copper, and 9.3 to 10.7% zinc with the balance being aluminum. This alloy is useful for brazing in vacuum or inert atmospheres from 1040 to 1112° F.
U.S. Pat. No. 3,703,763 describes forming a zinc bonding material using molten zinc to bond foamed aluminum with sheet aluminum.
In U.S. Pat. No. 5,422,191, an aluminum brazing alloy is described which can be used in either VB or CAB processes. The brazing alloy is clad with an aluminum alloy containing about 0.01 to 0.30% by weight lithium and 4 to 18% by weight silicon.
U.S. Pat. Nos. 5,232,788 and 5,100,048 describe an aluminum brazing method using silicon metal powder with a brazing flux such as potassium fluoroaluminate. The preferred metal component of the coating mixture is silicon, but other metals such as zinc, copper or nickel may be used.
A process for joining aluminum is described in U.S. Pat. No. 5,044,546 in which zinc is plated onto aluminum using a zinc immersion bath followed by cadmium plating and then heating in a vacuum to form a braze joint.
Another vacuum brazing process is found in U.S. Pat. No. 5,069,980 using two clad alloys comprising silicon and a small amount of magnesium. Other elements in the cladding may be at least one of the following from a group consisting of Pb, Sn, Ni, Cu, Zn, Be, Li, and Ge.
Another method of joining aluminum members is described in U.S. Pat. No. 5,316,206 where aluminum is coated with zinc or a 5% aluminum-zinc alloy by dipping into the molten alloy bath. Following preassembly and applying a flux material, the aluminum members were heated to an elevated temperature in a furnace to form braze joints.
In a prior art method of fluxless aluminum brazing, the aluminum parts being joined required plating with a braze-promoting layer typically comprising nickel and/or cobalt. The braze-promoting layer was applied by a variety of methods, including plating in alkaline plating media or by conventional electroless deposition from a hypophosphite solution. Alternatively, U.S. Pat. Nos. 3,970,237, 4,028,200, 3,553,825 and 3,482,305 describe plating baths for electroless and electrolytic plating of braze-promoting metals such as nickel, nickel-lead, cobalt, cobalt-lead or cobalt-nickel-lead onto aluminum alloy surfaces.
Presently there are several known fluxless brazing methods, as described in U.S. Pat. Nos. 3,332,517, 3,321,828 and many of the patents discussed above, which can be applied to brazing of aluminum alloys having a liquidus temperature somewhat above that of the presently available commercial Al—Si based filler metals (ie sufficiently above 577-635° C.). Unfortunately, many aluminum casting alloys including die castings, and some high strength heat treatable alloys (2xxx or 7xxx) have a melting temperature range below or very similar to those of the commercial brazing alloys, and therefore are not suitable for use with these brazing alloys. Also, as discussed, some of the prior art brazing methods are sensitive to magnesium concentrations above threshold amounts. This may limit their use in brazing aluminum alloys containing appreciable amounts of magnesium, such as 5xxx and 6xxx alloys.
Therefore, there is a continued need for brazing products and methods which are useful for brazing at low temperature in the absence of a flux.